Bryn Llewellyn <bryn@yugabyte.com> writes:
> The "at time zone" clause that can decorate a timetsamp[tz] value seems
> to allow an argument that’s an arbitrary expression that yields a value
> whose data type is "interval".
AT TIME ZONE is part of the SQL expression syntax, thus it's unsurprising
that its arguments can be arbitrary subexpressions.
> set time zone '-7 hours'::interval;
> brings a "42601: syntax error".
The SET statement, on the other hand, most definitely does not accept
expressions of any kind; only simple literals. That's in line with most
other utility commands in Postgres, but there are particularly good
reasons to be wary of trying to generalize SET. Cases such as
SET TRANSACTION ISOLATION LEVEL have to be able to execute without
a transaction context at all.
Yeah, it's asymmetric :-(. But the SQL committee has gifted us with
a pretty asymmetric language, plus there are various implementation
constraints that are not that easy to get rid of.
regards, tom lane